Modbus protocol: the cornerstone of industrial communication

In the wave of modern industrial automation, real-time data transmission and efficient management of equipment are crucial. Modbus protocol, as a pioneer of industrial communication protocol, has been one of the key technologies to achieve these needs since it was launched by Modicon in 1979.

The birth of the Modbus protocol marks the beginning of communication standardization in the field of industrial automation. Originally designed for communication between programmable logic controllers (PLCs), the Modbus protocol quickly gained market recognition for its simplicity and efficiency. Over time, the Modbus protocol has continued to evolve and has evolved into multiple variants to meet different communication needs.

1. Core Concepts

The core of the Modbus protocol lies in its master-slave communication model. In this model, the master device is responsible for initiating communication, and the slave device responds to the request. This design allows the system to effectively manage a large number of devices while keeping the communication clear and orderly.

(1) Equipment and machinery

Modbus devices include a variety of devices from simple sensors to complex controllers. These devices are connected through the Modbus protocol to form a highly integrated automation network.

(2) Data Model

The data model of the Modbus protocol is based on the concept of registers. Registers are units of data storage and are divided into two types: holding registers and input registers, which are used for data read and write operations and read-only operations respectively.

2. Communication Mode

The Modbus protocol supports multiple communication modes, including:

• RTU (Remote Terminal Unit): A binary, efficient communication mode suitable for serial communication.
• ASCII: A text-based communication mode that is easy to debug, but has lower transmission efficiency than RTU.
• TCP/IP: Supports long-distance network-based communications.
• UDP: A variation of the User Datagram Protocol that provides faster communication speeds.

Application areas:

The Modbus protocol is used in many fields such as industrial automation, vehicle systems, and IoT communications. In industrial automation, the Modbus protocol makes communication between devices simple and reliable. In vehicle systems, it is used to monitor and control key parameters of the vehicle. In the field of IoT, the TCP/IP variant of the Modbus protocol is particularly popular because it enables efficient communication between devices.

3. Modbus weaknesses

Although the Modbus protocol has achieved great success in the field of industrial communications, it also faces some challenges. One of the most important challenges is security. Since the Modbus protocol was not designed with modern network security requirements in mind, it lacks built-in security mechanisms and is vulnerable to network attacks.

With the rise of Industry 4.0 and smart manufacturing, the Modbus protocol needs to be continuously updated and improved to meet new security and performance requirements. At the same time, the Modbus protocol also needs to be better integrated with other emerging communication protocols such as MQTT and OPC UA to achieve wider applications.

4. Modbus protocol code implementation

There are many ways to implement the Modbus protocol, including Modbus ASCII, Modbus RTU, and Modbus TCP. The following is a sample code for implementing Modbus RTU and Modbus TCP clients using Python language and pymodbus library.

First, make sure the pymodbus library is installed:

bash

 pip install pymodbus

Modbus RTU Example Code

The following is an example of a Modbus RTU client that reads the holding registers of a slave device:

 from pymodbus.client.sync import ModbusSerialClient as ModbusClient # 创建Modbus RTU客户端client = ModbusClient(method="rtu", port="/dev/ttyUSB0", baudrate=9600, timeout=3) # 连接到Modbus服务器client.connect() # 读取保持寄存器address =0# 寄存器起始地址count =10# 读取寄存器的数量result = client.read_holding_registers(address, count) ifnot result.isError(): print("寄存器值:", result.registers) else: print("读取错误") # 断开连接client.close()

Modbus TCP Example Code

The following is an example of a Modbus TCP client that reads the input registers of a slave device:

 from pymodbus.client.sync import ModbusTcpClient as ModbusClient # 创建Modbus TCP客户端client = ModbusClient('127.0.0.1', port=502) # 连接到Modbus服务器client.connect() # 读取输入寄存器address =0# 寄存器起始地址count =10# 读取寄存器的数量result = client.read_input_registers(address, count) ifnot result.isError(): print("寄存器值:", result.registers) else: print("读取错误") # 断开连接client.close()

illustrate

(1) Modbus RTU:

• method="rtu": specifies the RTU mode.
• port="/dev/ttyUSB0": specifies the serial port device file, for example, it may be COM3 on Windows.
• baudrate=9600: specifies the baud rate.

(2) Modbus TCP:

• ModbusClient('127.0.0.1', port=502): specifies the IP address and port number of the server. The default Modbus TCP port is 502.

Functional Description:

• connect(): Establish a connection with the server.
• read_holding_registers(address, count): Reads holding registers.
• read_input_registers(address, count): Read input registers.
• isError(): Checks if there is an error.
• registers: Get the value of the register.

5. Comparison between Modbus and MQTT protocols

Modbus and MQTT are both communication protocols, but they are very different in design goals, application scenarios, and working principles. Here are some of the main differences between them:

Modbus:

• Application areas: Mainly used in industrial automation fields, such as communication between programmable logic controllers (PLCs), sensors and actuators.
• Communication model: Based on the master-slave model, a master device can communicate with multiple slave devices.
• Transmission media: Supports serial communication (such as RS-232, RS-485) and Ethernet communication.
• Data representation: Data is transmitted in binary or ASCII form.
• Message structure: has a fixed frame structure, including device address, function code, data and checksum.
• Security: Security was not considered when it was originally designed and there is a lack of built-in security mechanisms.
• Real-time: Provides determinism and real-time performance in industrial environments.

MQTT:

• Application areas: Suitable for a wide range of Internet of Things (IoT) applications, including smart homes, remote monitoring and mobile applications.
• Communication model: Based on the publish/subscribe model, messages are routed via topics.
• Transmission medium: mainly based on TCP/IP network.
• Data representation: Data is transmitted in lightweight XML or JSON format.
• Message structure: The message includes a fixed header, optional remaining length, optional message body, etc.
• Security: Supports multiple security mechanisms, such as TLS/SSL encryption and client authentication.
• Real-time performance: Although designed to be lightweight and low-latency, it may not be as real-time as Modbus when the network is unstable.

Modbus is a protocol designed for industrial environments that emphasizes determinism and real-time performance, but its security and flexibility are relatively weak.

MQTT is a protocol designed for the Internet of Things. It emphasizes lightweight and low power consumption, is suitable for a wide range of application scenarios, and has better security and flexibility.

In practical applications, Modbus and MQTT can be used in combination. For example, the data of Modbus devices can be sent to the IoT platform through the gateway via the MQTT protocol, thereby realizing remote monitoring and management of industrial equipment.